Well, the real issue is that they let use customize our voices. There's about 12 of them, and it would be REALLY expensive (especially since some of the voice samples are from pretty well-known actors. In Japan, at least) to record each and every line for each and every voice. Not to mention it'd bloat the game even larger.
So they let us use our imaginations.
Plus it can be kind of jarring when dialog appears that we would never say. There ARE a few instances where we choose what to say, and sometimes it's in "Squeenixish" and doesn't sound natural.